Sky & Telescope - March 2019 - 54

MARCH 2019 OBSERVING
Deep-Sky Wonders by Sue French
NW

The Roman brothers stand high these spring nights.
Back comes the Chief in triumph,
Who, in the hour of ﬁght,
Hath seen the Great Twin Brethren
In harness on his right.
Safe comes the ship to haven,
Through billows and through gales,
If once the Great Twin Brethren
Sit shining on the sails.
- Thomas Babington Macaulay, Lays of Ancient Rome, 1842

H

igh in our evening sky, the constellation Gemini depicts Castor
and Pollux, the great twin brethren of
Roman mythology. During their voyage
with Jason on the great ship Argo, a star
alit on each of their heads as a raging
storm calmed. When later mariners saw
dual ﬂames of electrical discharge, now
known as St. Elmo's Fire, dance on their

54

M A RCH 2 019 * SK Y & TELESCOPE

ship's masts and rigging, they said it
was the twins protecting them. A single
light was an ill omen - a visit from
Helen, the sister of Castor and Pollux,
who was the ruin of Troy.
The star Castor is an amazing triple
double, three sets of close pairs orbiting each other in an intricate ballet.
Through my 130-mm refractor at 37×,
brilliant Castor AB is dazzling white
with much dimmer C glowing orange
and very widely separated to the southsoutheast. Boosting the magniﬁcation
to 102× pries A and B apart with room
to spare. The A component looks white,
but its companion to the northeast
seems to have a touch of yellow.
Although that's as far as a telescope
can take us, our mind's eye introduces
us to the rest of the crew. Each visible

p Supernova 2015I was discovered by
T. Noguchi of Chiba Prefecture, Japan,
on February 5, 2015, in the spiral galaxy
NGC 2357. The Type Ia supernova
peaked at magnitude 14.0.

star is a close binary, a revelation made
clear by its spectra. All six components
are main-sequence stars heated by
hydrogen fusion in their cores. Components Aa and Ba are both spectral type
A and about 2 to 2½ times the mass of
our Sun, while Ca is a red dwarf. Their
inseparable companions (Ab, Bb, and
Cb) are also red dwarf stars, each about
half the mass of our Sun. The C pair is
the eclipsing binary YY Geminorum.
Its brightness drops 0.7 magnitudes
twice during each 19.5-hour orbit, each
eclipse taking 82 minutes. This incredible star system tantalizes us from a
distance of 52 light-years.
Southwest of Castor, we'll ﬁnd the
intriguing bipolar planetary nebula
NGC 2371/72. When discovered in
1785, William Herschel logged it as:

M A R K WA LTERS

The Great Twin Brethren

Sky & Telescope - March 2019

Table of Contents for the Digital Edition of Sky & Telescope - March 2019

Contents
Sky & Telescope - March 2019 - Cover1
Sky & Telescope - March 2019 - Cover2
Sky & Telescope - March 2019 - 1
Sky & Telescope - March 2019 - Contents
Sky & Telescope - March 2019 - 3
Sky & Telescope - March 2019 - 4
Sky & Telescope - March 2019 - 5
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Sky & Telescope - March 2019 - Cover3
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